Pit chain formation on Mars

Strings of depressions dotting the Martian
landscape indicate that seismic activity - marsquakes - may still be
reshaping the surface of the planet, according to Dr. David Ferrill of
Southwest Research Institute in a paper published in the October 10 issue
of GSA Today. These pit chains occur along dilational faults, partially
filled or open cavities that served as conduits for past groundwater flow.

"These faults could now serve as reservoirs for water or ice, making these
locations of potentially great interest to the scientific community
searching for signs of life on Mars," said Ferrill, a senior program
manager at SwRI®.

"Astrobiologists consider subsurface aquifer systems high-priority targets
for a potential Martian fossil record," said Danielle Wyrick, an SwRI
planetary geologist who co-authored the GSA Today article. "Detecting
underground water is difficult because current Mars data show only the
surface. Pit chains are easy-to-recognize features that give us clues to
what's going on below the surface, including prospective groundwater
systems."

Ferrill, Wyrick and their team reached these conclusions after comparing
high-resolution imagery of the surface of Mars with pit chains discovered
in Iceland, and conducting laboratory experiments to recreate the
processes
they believe formed the pit chains. The work was funded internally through
an SwRI initiative directed to Mars research.

"The pit craters are larger and better preserved on Mars than on Earth
because the surface erosion and higher gravity on Earth result in smaller
pits that are rapidly erased, sometimes within decades," said Ferrill. In
many areas of Mars, pit crater chains appear to be some of the youngest
features, postdating drainage channels, faulting and impact craters.
Using visible spectrum image data of Mars from the Thermal Emission Imaging
System on the Odyssey spacecraft, the team mapped pit crater outlines,
surface drainage channels and fault traces. Pit craters can be observed at
all stages of formation. The smallest pits have apparently flat floors with
surface textures similar to the surrounding topographic surface; the
steeper pit walls are smooth.

"We deduce that some of these pits are youthful, perhaps even actively
forming, because surface subsidence has not destroyed the original surface
of in-falling material," explained Ferrill. Laboratory physical analog
modeling also supports these observations. Based on analysis of Mars data,
scientists simulated slip on a normal fault using unconsolidated dry white
or dyed sand to represent Mars surface materials. Constant thickness
rigid wooden or aluminum plates, with or without an overlying layer of
cohesive powder, represented dilating fissures beneath the sand.
Scientists initially placed the plates edge-to-edge and created tabular
voids by progressively separating the plates to simulate fault slip.
"Our physical models reproduced most pit chain morphologies observed
on Mars," said Ferrill.